For improving the stability in case of short circuit and for increasing the load change performance of, e.g., IGBTs, there has been a tendency to replace the well-known metallization using aluminum (AlSi or AlSiCu) with a thick Cu metallization in the regime of 5 μm to 20 μm. This shall provide a higher performance of IGBT modules, enabling higher operating temperatures and a higher resistivity with respect to switching losses due to enhanced heat dissipation.
However, the application of Cu as a metallization bears some drawbacks. For example, Cu reacts with Si at temperatures as low as room temperature and results in Cu silicides. As an example, Cu3Si forms at room temperature. Hence, a barrier between the Cu metallization and the Si is required. Typically, a barrier layer of a material with a high melting point is provided, such as W, Ta, Ti, Mo, or an alloy of these elements with other elements, such as TiW or TiN. Frequently, combinations of various layers employing different elements are used.
Furthermore, Cu atoms tend to diffuse into Si and can thus drastically reduce the life time of minority charge carriers. This may lead to various drawbacks, for example an undesirable enhancement of the forward voltage and the leaking current in a blocking state.
Furthermore, in a humid atmosphere, an electrochemical reaction in the presence of a voltage difference can lead to a discharge of Cu ions. These are produced by anodic oxidation, may start wandering due to the presence of the electrical field during operation and may under certain circumstances accumulate at the cathode, whereby Cu dendrites are formed, which is also known as electro migration. The latter process occurs primarily in the region of the edge termination, because in this region a higher electric field is present. During the drift process towards the edge region, the positively charged Cu ions may disturb the well-defined change of potential.
Due to the required thorough isolation in the region of the edge termination, a TiW barrier as described above cannot be applied as a continuous protection in that region, because it would shortcut the device. Therefore, in the edge termination region, typically materials like SiO2 or Si3N4 are applied. The introduction of a barrier layer of a material like TiW for inhibiting an interaction between the Cu of a metallization layer and the Si of a semiconductor layer is for example known from K.-M. Chang et al., Journal of Applied Physics 82, 1469-1475 (1997).
Additionally, elements like field-ring or field-plate assemblies are applied in the edge termination region, for example comprising SiO2 or Polysilicon, which are also frequently combined in high voltage applications. An example of such a known field-plate field-ring assembly is shown in FIG. 1, having metallic field-plates 10 and doped field-rings 20 in the semiconductor structure. In the areas between metallic field-plates 10, the field lines typically leave the semiconductor structure, thus these regions have to be kept free of metal, with the result that the silicon oxide layers 15 are not protected in these regions. In the prior art, the metallization, including active electrode 26 and field-plates 10 is typically carried out using AlSiCu, having a Cu percentage of about 0.5% to 1.0%, whereby there is no reaction of the low Cu amount with the Si of the silicon. However, when replacing the AlSiCu with Cu, the above described degradation mechanisms can occur. In that case, the intermediate areas between field-plates 10 are potential soft spots with respect to the risk of intruding Cu ions, for example stemming from the field-plates 10, while a border region between well 20 and active electrode 26 is prone to the same problems.
For these and other reasons there is a need for the present invention.